Electrolyte creepage barrier for liquid electrolyte fuel cells

ABSTRACT

A dielectric member for electrically insulating a manifold or other component from a liquid electrolyte fuel cell stack wherein the dielectric member is adapted to include a barrier which chemically reacts with the liquid electrolyte to form solid products which are stable in the liquid electrolyte.

BACKGROUND OF THE INVENTION

[0001] This invention relates to dielectric members and, in particular, to dielectric members for electrically isolating manifolds and other components, from a liquid electrolyte fuel cell stack.

[0002] In a carbonate (liquid electrolyte) fuel cell stack with external manifolds for gas supply and discharge, the manifolds are electrically isolated from the fuel cell stack by a dielectric member in the form of a picture frame. The dielectric frame must be capable of operating at a voltage difference of between 100 and 1000 volts depending on the number of cells in a stack and the electrical configuration of the stacks when arranged in a power plant. A dielectric frame formed of Al₂O₃ has been found capable of providing electrical isolation to thousands of volts.

[0003] However, at the fuel cell operating temperature of 650° C., the liquid electrolyte in the fuel cell stack tends to creep over the surface of the dielectric frame. The frame and the stack are only separated by a thin porous gasket for gas sealing. The porous gasket becomes filled with electrolyte and as the dielectric frame comes in contact with the liquid electrolyte, the frame becomes wetted.

[0004] Once this occurs, a thin continuous layer of conductive liquid electrolyte film forms on the surface of the dielectric. Consequently, the electrical isolation provided by the dielectric frame can be compromised and can lead to stack malfunction. As a result, designers of these frames have looked to develop techniques to prevent or reduce the electrolyte creepage. The aim of these designers is to realize a dielectric frame able to provide stable long-term dielectric insulation of the liquid electrolyte fuel cell stack from the metallic manifold.

[0005] It is, therefore, an object of the present invention to provide a dielectric member which overcomes the above disadvantages.

[0006] It is a further object of the present invention to provide a dielectric member which exhibits increased resistance to dielectric creepage.

SUMMARY OF THE INVENTION

[0007] In accordance with the principles of the present invention, the above and other objects are realized in a dielectric member for electrically insulating a manifold or other component from a liquid electrolyte fuel cell stack wherein the dielectric member is adapted to include a barrier which chemically reacts with the liquid electrolyte to form solid products which are stable in the liquid electrolyte. In this way, the barrier inhibits flow or creepage of electrolyte from reaching the metallic manifolds.

BRIEF DESCRIPTION OF THE DRAWING

[0008] The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which:

[0009]FIG. 1 shows a fuel cell stack incorporating a dielectric member having a barrier in accordance with the principles of the present invention;

[0010]FIG. 2 illustrates schematically one form of the dielectric member of FIG. 1;

[0011]FIG. 3 shows a test configuration for testing a dielectric member having a barrier in accordance with the principles of the present invention;

[0012]FIG. 4 shows test results for a dielectric member of FIG. 3;

[0013]FIGS. 5 and 6 show further configurations for the dielectric member of FIG. 1; and

[0014]FIGS. 7 and 8 illustrate application of the invention to a dielectric frame.

DESCRIPTION OF THE INVENTION

[0015]FIG. 1 shows a fuel cell stack 1 in which a metallic manifold 2 abuts a face 1A of the stack. The manifold 2 can either serve to input gas or to extract gas from the stack 1.

[0016] Situated between the stack 1 and the manifold 2 are a gasket 3 and a dielectric member 4. The gasket 3 contacts the face 1A of the stack, while the dielectric member is situated between the gasket 3 and manifold 2. The dielectric member 4, typically, may have the form of a picture frame.

[0017] The dielectric member electrically isolates the metallic manifold 2 from the stack 1. As shown in FIG. 2, the dielectric member 4 includes a barrier 5 which is situated in the path of the liquid electrolyte flowing from the stack 1 through the gasket 3. In accordance with the invention, the barrier 5 is adapted to chemically react with the liquid electrolyte (e.g., carbonate electrolyte) of the stack 1 to produce solid products which are stable in the electrolyte. As can be appreciated, the production of these products inhibits flow of the electrolyte along the surface of the dielectric member. As a result, electrolyte creepage is reduced and the insulating characteristics of the dielectric member are preserved.

[0018] The material used for the barrier 5 can take on a variety of forms. One material found usable is calcium aluminate cement (Secar, available from LaFarge Corp.) At 650° C., Secar quickly reacts with Li₂CO₃ to form solid products consisting of LiAlO₂, CaO and K₂CO₃. These products are chemically stable in the liquid electrolyte (molten carbonate) environment. Another material is γ-Al₂O₃. Further common materials, such as MgAl₂O₄ powder and CaAl₂O₄ powder, can also be used.

[0019] The effectiveness of the above-mentioned materials as barriers depends not only on the chemical nature of the materials, but also on the amount of the material used. As long as there is sufficient reactive material, the electrolyte from the stack 1 will not creep over the entire surface of the dielectric member 4 so as to be able to reach the manifold 2.

EXAMPLE 1

[0020] A dielectric member using a barrier 5 comprised of Secar (mechanical mixtures of A1 ₂O₃ and CaO) was fabricated. The dielectric member comprised a grooved Al₂O₃ rectangular bar in a dimension of 4″×1″×0.625″ with Ra 29 surface finishing (Ra: the average deviation of the profile from the mean line, in μin). The Secar was embedded in grooves on both sides of the bar, as shown in FIG. 3.

[0021] The effect of Secar as a reactive barrier was then evaluated in an accelerated electrolyte pool test. In the test, the bottom of the dielectric member was submerged in a liquid electrolyte pool (infinite electrolyte supply), and a piece of gasket, serving as electrolyte absorbent, was laid on the top surface to collect the creeping electrolyte. The results from this test are shown in FIG. 4, and demonstrate that the production of reaction products caused significant delay in electrolyte creepage.

EXAMPLE 2

[0022] A dielectric member 4 as shown in FIG. 3 was formed in this case with the barrier comprised of γ-Al₂O₃ powder. This member was similarly tested as described in Example 1 and the results are also shown in FIG. 4. These results similarly indicate that the barrier caused significant reduction in electrolyte creepage.

EXAMPLE 3

[0023] In a liquid carbonate fuel cell stack, a dielectric frame as described in the U.S. Pat. No. 4,414,294 may be employed. This dielectric frame, as shown in FIGS. 7 and 8, includes straight segments 71 which are connected at joints. The joint area, shown as forming a keyway 72 in FIGS. 7 and 8, has the highest liquid electrolyte creepage due to increased creepage surfaces in the joint and possible capillaries formed between the straight bars and the connecting key 73 inserted in the keyway 72. A barrier 5 made of Secar cement, with a dimension 1.50″×0.625″×0.031″, was placed at the top of the joint area on the surface facing the manifold in a 250 kW molten carbonate fuel stack(340) cells. In approximately 12,000 hours of operation, the barrier partially reacted with the liquid electrolyte, and no electrolyte crossed the barrier and reached the manifold. This example confirmed the effectiveness of the use of a dielectric member provided with a chemically reactive barrier in actual fuel cell operation.

[0024]FIGS. 5 and 6 illustrate different configurations for the dielectric member 4 and the barrier 5. In the dielectric member 4 of FIG. 5, the barrier 5 comprises barrier inserts 5A and 5B which are embedded in the sides of the dielectric member. In the dielectric member 4 of FIG. 6, the barrier 5 comprises a layer situated on the surface of the member 4 facing the manifold 2.

[0025] The barriers 5 of the dielectric member 4 of the invention can be fabricated by various processes. Thus, the barriers 5 can be formed with high temperature ceramic binders using a painting or a casting process. They can also be formed by the standard tape casting technique. In both dielectric members 5 of FIGS. 5 and 6, the presence of the barrier 5 results in reducing electrolyte flow and, thereby prolonging dielectric life.

[0026] In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A dielectric member for use in electronically insulating a manifold or other component from a liquid electrolyte fuel cell, said dielectric member including a barrier which chemically reacts with the liquid electrolyte to form solid products which are stable in the electrolyte.
 2. A dielectric member in accordance with claim 1, wherein said barrier comprises one of CaAl₂O₄ powder, MgAl₂O₃ powder, Al₂O₃ powder and calcium aluminate cement.
 3. A dielectric member in accordance with claim 1, wherein said barrier is formed as a surface layer on a surface of the dielectric member.
 4. A dielectric member in accordance with claim 3, wherein said surface of said dielectric member faces said manifold.
 5. A dielectric member in accordance with claim 1, wherein said barrier member is embedded in the sides of said dielectric member.
 6. A dielectric member in accordance with claim 1, wherein said dielectric member has the shape of a frame.
 7. A dielectric member in accordance with claim 6, wherein said frame includes a plurality of segments joined at a joint and said barrier is situated in the area of said joint.
 8. A dielectric member in accordance with claim 7, wherein said joint includes a keyway area for receiving a key for joining said segments.
 9. A liquid electrolyte fuel cell system comprising: a liquid electrolyte fuel cell stack; a manifold member abutting a surface of said liquid electrolyte fuel cell stack; a dielectric member situated between said manifold member and said surface of said liquid electrolyte fuel cell stack, said dielectric member including a barrier which chemically reacts with the liquid electrolyte to form solid products which are stable in the liquid electrolyte.
 10. A liquid electrolyte fuel cell system in accordance with claim 9, wherein: said liquid electrolyte is carbonate.
 11. A liquid electrolyte fuel cell system in accordance with claim 9, wherein: said barrier members comprise one of CaAl₂O₄ powder, MgAl₂O₃ powder, Al₂O₃ powder and calcium aluminate cement.
 12. A liquid electrolyte fuel cell system in accordance with claim 9, wherein: the barrier is formed as a surface layer on a surface of the dielectric member.
 13. A liquid electrolyte fuel cell system in accordance with claim 12, wherein: said surface of said dielectric member faces said manifold.
 15. A liquid electrolyte fuel cell system in accordance with claim 9, wherein: said barrier member is embedded in the sides of said dielectric member.
 16. A liquid electrolyte fuel cell system in accordance with claim 9 further comprising: a gasket situated between said dielectric member and said surface of said fuel cell stack.
 17. A liquid electrolyte fuel cell system in accordance with claim 9 wherein: said dielectric member has the shape of a frame.
 18. A liquid electrolyte fuel cell system in accordance with claim 17, wherein: said frame includes a plurality of segments joined at a joint and said barrier is situated in the area of said joint
 19. A liquid electrolyte fuel cell system in accordance with claim 18, wherein: said joint includes a keyway area for receiving a key for joining said segments. 